1. Field of the Invention
The present invention relates to method for the direct heat treating an austenitic stainless steel wire rod. An object is to provide a method for direct heat treating an austenitic stainless steel wire rod suitable for further cold working by utilizing the retained heat of a hot rolled austenitic stainless steel wire rod emerging from the final finishing stand of a hot rolling mill.
2. Description of the Prior Art
The invention stems from an observed fact as follows. A conventional austenitic stainless steel wire rod produced by the hot rolling process is available for extensive use in various fields, such as, in making nails, rivets, wire nets, and other miscellaneous small parts. In accordance with a particular use, the wire rod is subjected to further cold working, such as, wire drawing to a desired size.
The common wire rod is manufactured by the steps of hot rolling, reeling into a coil form, and cooling, but the wire rod thus manufactured as such has so fine a crystal grain that its strength is too high to be suitable for further cold working. Further, chromium carbide precipitates on the crystal grain boundary, which may cause corrosion. Accordingly, a hot rolled austenitic stainless steel wire rod is usually heated at a temperature of 1050.degree.-1100.degree. C. so as to grow the crystal grains to a suitable degree, and subjected to a so-called solution heat treatment in which chromium carbide is soluble in the matrix and then quenched, and further, subjected to cold working.
Japan Open-laid Pat. No. SHO50-96419(1975) discloses the above so-called direct heat treatment of a wire rod in which the solution heat treatment described above is conducted on the rod by taking advantage of the retained heat of the rod, which has just emerged from hot rolling. The method of this disclosure comprises the steps of reheating the wire rod having a temperature of about 1000.degree. C. which has just emerged from the final finishing stand of the hot rolling mill to a solution heat treatment temperature in a reheating furnace equipped before or after a reeler, and then quenching it. However, this method needs reheating, so let us say that a conventional solution heat treatment arrangement is simply installed at the rear of a hot rolling line. In other words, reheating for a relatively extended period of time is required in order to grow the crystal grains of the wire rod for the purpose of imparting excellent cold working properties, therefore beneficial and gratifying effects cannot be expected.